Joined body of pyrolytic graphite and metallic members

ABSTRACT

A method for joining pyrolytic graphite to metallic members by using a brazing composition comprising at least 90 percent by weight of a copper-chromium, gold-chromium, or silver-chromium mixture.

'7 Patent 1 1 I 371379fi Takamori et al. 1 1 Jan. 30, 1973 1/ 0 3 1 541IOINED BODY OF PYROLYTIC GRAPHITE [56] References Cited 1 l i I ANDMETALLIC MEMBERS UNITED STATES PATENTS [7 5] Inventors: TakeshiTakamori; Masanobu 3122424 2/1964 m g .19 195 5 Akanuma bmh of T Japan3,356,468 12/1967 Pinter "29 /195 I [73] Assignee: Nippon ElectricCompany, Limited, 3,425,116 2/1969 Crooks et a1. .29/195 X Tokyo-to,Japan 3,442,006 5/1969 Guichet at al. ..29/195 X Filed g 12 1969 v 13,497,332 2/1970 Donneily et ai. v .29/195 v i v A i P 849,463 PrimaryExaminer-L. Dewayne Rutledge Assistant Examiner-E. L. Weiss [30] ForeignApplication Priority Data Atl0rney-0strolenk, Faber, Gcrb & Soffen Aug.14, 1968 Japan ..43/57883 [57] ABSTRACT [52] U.S.Cll ..29/l9gglgllfsl3dll1 A mehod for joining pyrolyfic graphite to metallic C 2 1members by using a brazing composition comprising at least 90 percent byweight of a copper-chromium, gold-chromium, or siiver-chromium mixture.

1 Claim, 1 Drawing Figure JOINED BODY OF PYROLYTXC GRAPHITE AND.METALLIC MEMBERS This invention relates to a joined body comprising apyrolytic graphite member and a metallic member and a method of joiningthose members into a joined body which is particularly useful atelevated temperatures of at least about 700 C.

For the purpose of the invention, the term pyrolytic graphite as hereinused means the known polycrystalline material formed by pyrolyzing oneor more different types of carbonaceous gases (i.e., gases containingcarbon) at a reduced or normal pressure or in the presence of anothergas or gases, to the extent that the deposition of the carbon is causedonto a heated surface of a substrate. The material usually has a layerstructure, the layers being formed parallel to the heated surface onwhich they are deposited. Since the bonding strength of the adjoininglayers in the direction perpendicular to the layer plane (or in thedirection of axis'c as it is called hereinafter) is significantly lowerthan that along the layer plane (or in the direction of axis a as it iscalled hereinafter), flaking is likely to occur along the layer plane.To obtain firm joining of such pyrolytic graphite material with ametallic member, it is essential to use a surface parallel to the axis cof the material as the joining surface, and to choose a brazing materialthat can impart an acceptable wetting property or brazeability on thejoining surface of the material and also on that of the metallic member.Because pyrolytic graphite has an extremely low thermal expansioncoefficient in the direction of axis a in sharp contrast to a very highcoefficient in the direction of axis c,joining of such graphite with ametallic member would result in a fairly great internal stress withinthe joined portion of the two members. in this case a desirablecondition for brazing is that the weakest point of the joined portionlies in the pyrolytic graphite, and not in the brazing metal nor in theinterface between the brazing metal and the graphite. In other words,the strongest joined body of pyrolytic graphite and metallic members isone whose strength depends on the strength ofthe pyrolytic graphite. Toobtain a firmly joined body, the brazing materials are required to have,in addition to the properties above mentioned, sufficient ductility torelieve the internal stress in the joined portion to some extent.Numerous varieties of brazing materials have heretofore been proposedand tried for joining of the socalled artificial graphite members formedby shaping and sintering carbonaceous powdery materials through the useof conventional methods, with metallic members or with other artificialgraphite members. However, none has proved fully satisfactory for thepurpose of joining pyrolytic graphite with metallic members. Forexample, alloys of iron with smaller proportions of nickel or chromiumare in many cases employed for joining artificial graphite with metals.Investigations by the inventors have revealed that these alloys are notsuitable for brazing between pyrolytic graphite and metal because theirwettability with respect to the surface parallel to the axis ofpyrolytic graphite is not quite desirable. In other instances of brazingof artificial graphite to metals, silicon and silicides are used as thebrazing agents. These agents have also been found inadequate for thesame purpose as above, according to the investigations by the inventors,because they are not merely too brittle as such but, when they melt incontact with the surface parallel to the axis 0 of pyrolytic graphite,they react vigorously with the pyrolytic graphite, often resulting inthe formation of B- SiC on the joining surface and fracture of thepyrolytic graphite along the layer plane deeply extending into thegraphite. ln joining an artificial graphite member with a titanium ofzirconium member, it is sometimes the practice to employ a brazing metalconsisting essentially of one or more members of the group consisting ofcopper, silver and gold. if such a brazing metal is utilized in thebrazing of pyrolytic graphite to a metallic member, a considerablystrong joined body will be ob tained provided that a-suitable amount oftitanium or zirconium is diffused into the joining surface of thepyrolytic graphite. Actually, however, the amount and geometry of thebrazing material, the brazing temperature, and other pertinent factorsare combined delicately to influence the strength of the resulting 0joined body, thus making it difficult to obtain stabilized 3O firmlyjoined body of pyrolytic graphite and metallic member, and a brazingmaterial sandwiched in conditions. Moreover, the metals with which thepyrolytic graphite is joined are limited to titanium, zirconium or theiralloys. Among other proposals is that of joining pyrolytic graphite andartificial graphite members with the aid of titanium carbide, zirconiumcarbide, titanium or the like. As will be naturally obvious, thesebrazing materials can hardly be used in joining pyrolytic graphite withmetals.

' it is an object of the present invention to provide a metallicmembers. I

Another object of the invention is to provide brazing materials chosenfor joining of pyrolytic graphite member with metallic member.

Yet another object of the invention is to provide a novel method offorming a firmly joined body of a pyrolytic graphite member and ametallic member.

According to this invention, there is provided a joined body comprisinga pyrolytic graphite member, a

between said two members for joining together of the rtwo members andwherein said brazing material is an alloy of a combination ofcompositions with a total proportion of not less than 90 percent byweight selected from a group of combinations of compositions consistingof combinations of copper-chromium compositions wherein the chromiumcontent ranges from 10 to 75 percent by weight, combinations ofsilver-chromium compositions wherein the chromium content ranges from 10to 85 percent by weight, and combinations of gold-chromium compositionswherein the chromium content ranges from 10 to percent by weight. Thematerials for joining together the pyrolytic'graphite member and themetallic member may be an alloy composed principally of one of the abovecombinations. Alternatively, metals within a certain combination may bearranged in the form of sheets stacked by lamination, cladding orelectroplating. In the former, the temperature for the joining operationis selected at a suitable temperature not lower than the melting pointof the brazing alloy. In the latter, the joining is carried outsimultaneously with alloying the metal sheets by heating to atemperature higher than the melting point of 5 either metal in thecombination that has the lower melting point. ln either case, thejoining should be performed in a non-oxidizing atmosphere such as in avacuum.

Experiments by the inventors indicated that any of the combinations ofmetals for brazing whose chromiurn content is outside the range abovespecified, normally fails to give a satisfactorily joined body. Thiswill be further demonstrated in connection with some examples of thepresent invention. Up to this writing, however, it has not been clearwhy the above limitations to the materials for joining pyrolytic memberwith metallic members serve advantageously for the purpose of obtainingstrongly joined bodies. Presumably the reason for this is thepossibility that the cooperation of the reactivity between chromium andpyrolytic graphite and the ductility of copper, silver or gold gives afavorable effect upon the wettability, binding power,

and the like, against the pyrolytic graphite.

Now, the wettability of the brazing materials used in the estimation ofthis invention with respect to pyrolytic graphite will be describedbelow.

The sole FIGURE in the accompanying drawing is a graph showing thewetting properties of alloys covering the composition ranges of brazingmaterials used in the invention. in the individual combinations, theproportions of chromium added to copper, silver or gold are plotted inpercent by weight on the abscissa against the contact angles with thesurface parallel to the axis c of pyrolytic graphite on the ordinate. inthe graph, each plot represents contact angles which werephotographically determined by placing an alloy piece on a small plateof pyrolytic graphite and then melting the same in an atmosphere ofargon. It will be appreciated that within the composition ranges ofbrazing materials used in the invention the individual alloys exhibitvery good wettability for pyrolytic graphite. Incidentally, the inventors study with similar experiments revealed that the brazingmaterials of Fe alloys containing chromium or nickel, which have oftenbeen employed in brazing of artificial graphite, always have contactangles of more than 40 degrees with respect to pyrolytic graphite.

Regarding the binding power exerted between the pyrolytic graphitemember and metallic member of the joined body according to theinvention, description will be made hereunder in conjunction withspecific exam ples.

it will be readily understood by those skilled in the art that, where apyrolytic graphite member and a metallic member are to be joinedtogether with the use ofa suitable metal as the joining agent or brazingmetal, the fracture strength of the joined body thus obtained isgenerally dictated, because of the sufficiently high strength of themetal member and the strength of the interface of the metal member andthe brazing metal, by the strength of the pyrolytic graphite member, bythe strength of the interface of the pyrolytic graphite and the brazingmetal, or by the strength of the portion of the brazing metal which hasbeen more or less carburized by the presence of the pyrolytic graphite.As described above, it is most desirable that the fracture strength ofthe joined body is solely dependent upon the fracture strength of thepyrolytic graphite member. Thus,.to exemplify the present invention, thethree occasions above itemized with the combinations covering thecomposition ranges of brazing metals used in the invention werecompared. The results were as shown in Tables 1 to 3. In each of theseexperiments, the op posed faces of two pyrolytic graphite pieces whichwere parallel to the axis 0 of each pyrolytic graphite piece were joinedtogether with a brazing metal of the invention, then the joined portionwas loaded with a force exerted by a steel wedge in the directionparallel to the interface between brazing metal and pyrolytic graphite,and then the location and appearance of the fracture were observed. Theobserved results were classified with three symbols, A, B, and Caccording to the location of the fracture. A indicates that thepyrolytic graphite was fractured, B indicates the fracture at thebrazing metal portion, and C indicates the fracture at the interface.Table 1 shows the fracture conditions where various combinations ofcopper and chromium were used as brazing metals. in all of theseexperiments a constant brazing temperature of l,470 C. was employed.Table 2 shows the fracture conditions where various combinations ofsilver and chromium were used as brazing metals, all ata brazingtemperature of 1460" C. Table 3 shows the fracture conditions observedwith various combinations of gold and chromium as brazing metals, all ata brazing temperature of 1520 C. Throughout the experiments the joiningwas conducted in an argon atmosphere in an electric furnace. Nodifference in the location of fractures was observed between the twoforms of the brazing metal; one is an alloy of various combinations andthe other consists of sheets of those metals stacked with or withoutcladding or electroplating.

TABLE 1 Composition (wt.%) Cu Cr Location of fractures 190 0 C75 A7129 A6139A54 46 A 48 52A4258A3268A2278B1684B TABLE 2 Composition (wt.%)Ag CrLocation of fractures 100 0 C 87 13 A 77 23 A 71 29 A 65 A 6238A4852A3862A3070A 1981A 135878 TABLE 3 Composition (wt.%) Au CrLocationof fractures 10% 0 C 8119 A 7129 A 60 40 A 50 A 40 A 30 A it should beobvious from Tables 1 to 3 that, the combinations of copper, silver,gold and chromium as brazing metals have beneficial effects providedthat they are within the composition ranges claimed by the presentinvention. Although only the combinations of two kinds of metals havebeen given in the above examples, it has now been found that theaddition of one or more different kinds of metal to the saidcombinations is generally permissible only if the additive amount doesnot exceed 10 percent by weight on the basis of the weight of the parentcomposition. Such addition may be made without departing from the spiritand scope of the invention. As regards the atmosphere for the brazingpurposes, an inert gas other than argon, a

vacuum, or other atmosphere which is at least non-oxl. A joined bodycomprising:

a. a pyrolytic graphite member; I the alloy;

a metallic member, and 2. Silver-chromium alloy wherein the chromiumcona joining member Sandwiched in the tent ranges from 10 to 85 percentby weight ofthe members (a) and (b) for joining together said n andvmembers (a) and (b); 5 3. Goldchromium alloy wherein the chromiumconsaid "pining member (c) comprising an alloy with a total proportionof not less than 90 percent by weight selected from a group of alloysconsisting of:

l. Copper-chromium alloy wherein the chromium content ranges from lOto75 percent by weight 0f 10 tent ranges from 10 to 75 percent by weight;the

remainder of the components of said alloy being one or more differentkinds of metal.

2. Silver-chromium alloy wherein the chromium content ranges from 10 to85 percent by weight of the alloy; and
 3. Gold-chromium alloy whereinthe chromium content ranges from 10 to 75 percent by weight; theremainder of the components of said alloy being one or more differentkinds of metal.